1. Technical Field
The present invention relates to internal engine ignition systems. More particularly, it relates to a method of estimating MBT timing using in-cylinder ionization current.
2. Discussion
The prior art includes a variety of conventional methods for detecting and using ionization current in a combustion chamber of an internal combustion engine. However, each of the various conventional systems suffers from a great variety of deficiencies. For example, prior art ionization current detection circuits are generally too slow and generate a current signal with low signal-to-noise ratio.
It is a goal of every ignition system of an internal combustion engine to time the ignition/spark so that the engine produces its maximum brake torque with a given air to fuel mixture. This ignition/spark timing is referred to as the minimum timing for best torque timing. So far, the minimum timing for best torque timing control strategies appearing in the prior art are predominantly based on post flame peak detection. The post flame peak usually lines up with the peak pressure location. It has been recognized that the MBT timing occurs when the peak pressure location is around 15° After Top Dead Center. By advancing or delaying the spark timing until the second peak of the ionization signal peaks around 15° After Top Dead Center, it is assumed that the minimum timing for best torque timing is found. Unfortunately, the second peak of the ionization signal does not always appear in the ionization signal waveform at all engine operating conditions. At light loads, lean mixtures, or high exhaust gas re-circulation rates, the second peak can be difficult to identify.